1. Field of the Invention
This invention relates to a lens moving mechanism suitable for a photo-taking lens with an auto focus function in which an optical lens unit is finely moved in the direction of the optical axis thereof to thereby effect the detection of an in-focus state.
2. Related Background Art
As the auto focus device of a video camera, there is known a so-to-speak mountain climbing system in which the degree of minuteness of a photographing image field is detected from a high frequency component in an image signal and the position of a lens is controlled so that the degree of minuteness may become maximum. This system is described in detail in Japanese Laid-Open Patent Application No. 56-51164, and this system will hereinafter be described briefly with reference to FIG. 6 of the accompanying drawings. In FIG. 6, the reference numeral 100 designates a lens, the reference numeral 101 denotes a signal processing portion, and the reference numeral 102 designates a motor.
The light of an object entering the lens 100 is converted into an electrical signal by the signal processing portion 101. By the utilization of the fact that the high frequency component of this electrical signal is small if the in-focus state of the lens 100 is insufficient, and becomes greater as the lens is more in focus, the signal processing portion 101 causes the converted electrical signal of an image pickup device to pass through a high-pass filter, whereafter it detects and integrates the electrical signal over a period during which an image field is formed, i.e., a field period (in the case of a television system, 1/60 sec.), and detects the state of the focus.
Then, a focus lens is moved by the motor so that a high frequency signal may be obtained, and there is obtained the in-focus state.
Now, when detecting the direction of out-of-focus, it is usually practiced to finely move (wobble) some lens in the photo-taking lens 100 in forward and backward directions along the optical axis thereof.
Heretofore, in such a zoom lens device, as a lens moving mechanism for finely moving a lens unit, use has usually been made of a system as shown in FIG. 7 of the accompanying drawings. In FIG. 7, the reference numeral 103 denotes a finely moved lens unit, the reference character 103a designates a holding member holding the lens 103, and the reference numeral 104 denotes the fixed portion (fixed barrel) of a lens device body. The lens holding member 103a is fitted in an aperture (not shown) formed in the fixed portion and is supported for rectilinear movement in the direction of the optical axis. The reference character 105a designates the external thread of a feed screw directly connected to the motor, and disposed parallel to the optical axis and engaged with an internal thread 103b provided on the lens holding member 103a.
Under such a construction, the lens 103 is finely moved by the rotational operation of a motor 105 and the in-focus direction is detected by the signal processing portion.
Now, in this system, out-of-focus is caused by the fine movement of the lens unit and therefore, in a lens device for photographing, and particularly a TV or video lens device handling moving images, it is necessary to suppress the out-of-focus within a range in which a variation in an image signal is detectable and moreover to such a degree that it is not conspicuous to the eyes. Also, after the detection by wobbling, a focus lens unit is moved and therefore, to realize auto focus quick in response, it is important to effect the wobbling at high speed.
Here, the amount of variation in the focus by the wobbling is determined by the amount of movement of the lens unit 103, but the amount of movement of the lens unit 103 which satisfies the above-described condition is related to the depth of focus. That is, when the depth of focus is shallow, the variation in the focus will be conspicuous unless the amount of movement of the lens unit 103 is made small, and when the depth of focus is deep, the variation in the focus cannot be detected unless the amount of movement of the lens unit 103 is made great. On the other hand, the depth of focus greatly varies depending on the focal length of the zoom lens and the F value of the aperture, and the difference between the time when the depth of focus is deepest and the time when the depth of focus is shallowest is on the order of several tens of times. Accordingly, the amount of movement of the lens unit 103 by the wobbling must also be varied as much as several tens of times between the minimum and the maximum.
Heretofore, however, in the lens moving mechanism used for wobbling, use has been made of a feed screw in which the amount of feed relative to the rotation of the motor 105 is constant and therefore, it has been necessary to make the total number of revolutions of the feed screw for moving the lens unit small when the depth of focus is shallow, and very great when the depth of focus is deep. Therefore, to effect the wobbling when the depth of focus is deep, it has been necessary to rotate the motor a lot and thus, much time has been taken until the end of the wobbling and after all, this has led to the problem that the response of auto focus becomes slow. Conversely, if the lead of the feed screw is made great so that the lens unit can be moved quickly when the depth of focus is deep, the number of revolutions of the motor when the depth of focus is shallow becomes very small and therefore, it has become difficult to move the lens unit with good accuracy.
On the other hand, in the lens device for photographing, as a mechanism for moving a lens unit for optical tracking adjustment or the like in the direction of the optical axis thereof, use is made of one as shown in FIGS. 16A and 16B of the accompanying drawings. In these figures, the reference numeral 71 designates a movable lens unit, the reference numeral 72 denotes a lens holding member holding the lens 71, and the reference numeral 73 designates the fixed portion (fixed barrel) of a lens barrel body which corresponds to an outer cylinder, and the lens holding member 72 is fitted in an aperture formed in the fixed portion 73 and is rectilinearly movable in the direction of the optical axis. The reference numeral 74 denotes a ring member having a cam slot, and fitted to the outer periphery of the fixed portion 73 and rotatable about the optical axis. The reference numeral 75 designates a pin member mounted on the lens holding member, and fitted in a straight groove formed in the fixed portion 73 and the cam slot of the ring member 74.
In the construction as described above, when as shown in FIGS. 16A and 16B, the ring member 74 is manually rotated or electrically rotated by the use of a motor or the like, the pin member 75 moves along both of the cam slot of the ring member 74 and the straight groove of the fixed portion 73, and the lens holding member 72 on which the pin member 75 is mounted rectilinearly moves in the direction of the optical axis. The movable lens unit 71 is then rectilinearly moved in the direction of the optical axis, whereby the intended optical lens adjustment is achieved.
In the above-described example of the conventional art, however, the lens holding member 72 rectilinearly moves while frictionally sliding by the aperture in the fixed portion 73 and the fitted portion and therefore, for the lens holding member 72 to move smoothly with a small force, it is necessary that there be a suitable gap in the fitted portion between the lens holding member 72 and the fixed portion 73. This has led to the possibility that as shown in FIG. 17A of the accompanying drawings, the lens holding member 72 becomes eccentric or as shown in FIG. 17B of the accompanying drawings, an inclination is caused to the lens holding member 72 by backlash and the optical performance is deteriorated.